To optimally design targeted interventions aimed at reducing substance use, one must know which characteristics or traits are involved in substance use and how these characteristics lead to substanceuse; these relations are likely to be complex and to involve bi-directional influences. The present project focuses on two separable traits related to substance use: reward seeking, defined as a tendency to enjoy and pursue exciting activities and new experiences, and low inhibition, defined as the tendency to act without thinking or reflection. The research primarily examines basic, elemental processes (e.g., risk tolerance, inhibitory control, delay discounting, Behavioral Inhibition or Activation System functioning) that may mediate the relations between these traits and substance use. The elemental processes are hypothesized to reflect the activity of two biological systems: an approach/reward seeking system and an inhibitory system. The reward system is theorized to be responsive to reward and is responsible for activating approach behavior. The inhibition system is theorized to be sensitive to punishment and is responsible for inhibiting ongoing behavior and initiating active information processing. Several potential social variables are also examined as potential mediators (i.e., substance use expectancies and peers). The research also allows for the possibility that substance use itself changes the traits and the underlying elemental processes. Finally, the project examines the interrelations among the elemental processes themselves. All of these issues are examined in the context of a three-year, two cohort, longitudinal study. There are four specific aims: 1. To identify the mechanisms underlying the relations leading from reward seeking and inhibition to substance use. Inhibition and reward seeking are distinct traits with distinct outcomes including substance use and abuse. Little is known, however, about the processes that link these traits to their outcomes. 2. To examine the effect of substance use on the processes associated with reward seeking and inhibition. Research has shown acute effects of drug exposure on the processes under study. For example,alcohol has been shown to have acute effects on inhibitory control. Additionally, a number of studies have demonstrated process deficits in substance abusers. Thus, it is possible that drug use influences the processes underlying reward seeking and inhibition, although this has not been demonstrated longitudinally. 3. To examine the relations among the underlying processes themselves. The processes examined in the present research come from a variety of levels and modes of assessment, ranging from elementary inhibitory control through Behavioral Inhibition System functioning assessed via psychophysiology. Although each task and measure is related to substance use and has been designed to assess one of the two hypothetical systems, these tasks have not been examined together previously.